Today's business communications environment consists of two separate network infrastructures: a voice network (such as a private branch exchange (PBX)) characterized by real-time, high-reliability, constant bit-rate (CBR) connections; and a data network (such as a packet network) characterized by high-bandwidth variable bit-rate (VBR) connections. Business needs for simplified maintenance, management, and access to information on diverse networks are forcing the convergence of these networks along with a new class of real-time multimedia networks. Asynchronous transfer mode (ATM) provides a single infrastructure that cost-effectively and flexibly handles both switching and transmission for the traffic types mentioned above (voice, video, and data) for both local-area networks and wide-area networks. The evolving network convergence requires the adaptation of the legacy PBX voice traffic to ATM. Voice telephony over ATM (VTOA) specifications allow adaptation of compressed or uncompressed voice pulse-code modulated (PCM) data streams into streams (virtual circuits) of CBR cells.
An ATM cell, regardless of the traffic it carries, is a packet 53 octets long: 48 octets of payload attached to a 5-octet header. The header contains addressing and management information used to direct the cell from source to destination and to ensure that the negotiated aspects of the traffic-flow through the ATM network are met. CBR traffic is assembled into cell payloads using ATM Adaptation Layer 1 (AAL1). The AAL1 cell constructor layer uses the first octet of the payload for its header and the remaining 47 octets to carry CBR information. Once every eight cells, the AAL1 structured data transfer (SDT) cell constructor layer introduces a one-octet pointer into the CBR payload. This pointer designates traffic-block boundaries. It is used at the receiving end to generate framing signals for devices that convert the ATM traffic into T1 or E1 (telephony trunk) traffic. ATM cell construction is then completed by attaching the ATM header to the payload.
Known existing devices that convert the ATM traffic into T1 or E1 traffic do not synchronize (align) the received blocks of traffic with the appropriate bits on a T1, E1, or TDM bus that is synchronized to another source (e.g., the destination synchronization source).